How are you running your backend? There is a chance of print() statements being buffered or debug=False parameter messing up the stdout as it seems like you are running it in production mode. In such cases the procedure is to:

• enable dev (debug) mode - debug=True in the last line
• try print("something", flush=True) to bypass the buffering

If it returns status 200, that means the controller has been found and returns a response, so it has something to do with the IO mechanisms.